This invention relates to a method and apparatus for filtering molten solder, and in particular for removing soluble impurities from solder.
Solder is a tin-lead alloy comprised of from about 2 to 70 weight percent tin and the balance lead. Impurities in the solder can affect soldering performance and must be kept to a minimum. Standards for solder alloys set maximum tolerable impurities in alloy as provided by a supplier or refinery. Impurities can be inadvertently picked up during normal usage of the alloys, especially when solder pots with recirculation systems and passage of components through the molten materials are used. Impurities present in sufficient quantities can affect wetting properties, flow within the joint, melting temperature of the solder, strength capabilities of joints, and oxidation characteristics of the solder alloys.
For example, a molten tin-lead solder bath is capable of dissolving copper at a high rate, easily reaching 0.4 weight percent copper and higher. Copper in liquid solder can settle to the bottom of a solder bath as an intermetallic compound sludge, or precipitate in the solder as particles of the copper-tin intermetallic compounds. In addition, tin oxide particless, known as dross, can form in solder exposed to air. Such particles can detrimentally affect the solder bonding process or joining apparatus by precipitating, or accumulating the precipitate into larger agglomerations and depositing on the soldered parts or on the soldering apparatus.
Additional information about soldering and molten soldering apparatus such as wave solder pump systems can be found in, American Metals Handbook, Ninth Edition, Vol. 6, 1983, pp. 1069-1101, incorporated herein by reference.
It is an object of this invention to provide a method and apparatus for filtering particles from molten solder.
It is another object of this invention to provide a method and apparatus for removing soluble impurities in solder to an impurity level below which precipitates of the impurity do not form at soldering temperatures.